2 Answers
2

Does PageSpeed Insights use a real device? PageSpeed Insights' analysis does not use real devices. PageSpeed Insights fetches a site
with a webkit renderer (the same rendering engine that powers Chrome
and Safari) that emulates both mobile device and desktop devices.

Therefore, screen width should not be a factor as it will render accordingly in WebKit.

To figure this out I ran Page Speed Insight (PSI) on a page with ranges of media queries that changed the background color at different breakpoints. After narrowing the width down to within 20px, I added a media query for every width within that 20px range until I found exactly the width PSI was using.

The reason this is (arguably) important is, for me, I was attempting to get my portfolio site a 100 PSI score as an exercise. My smallest media query is max-width="600px". I was using responsive images with picturefill setting the smallest image in the srcset to the largest the images would appear at my smallest breakpoint (600px + 80px padding = 520px wide images) but was getting an awful PSI score. Adding a smaller, 320px wide image to the srcset fixed the score.

The reason I say arguably is that, for real-world usage, my page was optimized perfectly without caring about PSI's viewport width. But my goal was specifically to get a high PSI score therefore it was important to figure this number out.